Titanium alloy having high strength and super-low elastic modulus

ABSTRACT

The present invention relates to a titanium alloy which is capable of nonlinear elastic deformation and simultaneously has super-high strength, super-low elastic modulus and stable super-elastic properties. The titanium alloy according to the present invention comprises Nb, Zr and O as alloy elements, the remainder being Ti and inevitable impurities, and has a valance electron ratio (e/a) of 4.17 to 4.22, a Mo equivalent (Mo eq ) of 7.50 to 9.72, and an Al equivalent (Al eq ) of 1.42 to 14.53.

TECHNICAL FIELD

The present invention relates to a titanium alloy exhibiting nonlinear elastic deformation and having ultrahigh strength, ultralow elastic modulus and stable superelastic properties at the same time.

The present invention relates to a titanium alloy which has a strength of 1000 MPa or more and an elastic modulus of 60 GPa or less and, at the same time, exhibits nonlinear elastic deformation (i.e., the coefficient of correlation of a decrease in superelastic elongation (%) to an increase in oxygen concentration (mass %) of the titanium alloy is −0.5 (%/mass %) or more), and which has ultrahigh strength, ultralow elastic modulus and stable superelastic properties at the same time.

The present invention relates to a titanium alloy which does not contain elements toxic to the human body, such as aluminum (Al), vanadium (V) or nickel (Ni), and tin (Sn) having low corrosion resistance in vivo, and which comprises only titanium (Ti), niobium (Nb), zirconium (Zr) and oxygen (O), which are harmless to the human body, in which the titanium alloy has ultrahigh strength, ultralow elastic modulus and superelastic properties at the same time.

The present invention relates to a titanium alloy which does not contain heavy tantalum (Ta) having a high melting point (3,017° C.) or contains a small amount of Ta so as to prevent composition non-uniformity from being caused by tantalum (Ta) during melting and solidification, is light in weight, exhibits nonlinear elastic deformation making mass production possible, and has ultrahigh strength, ultralow elastic modulus and superelastic properties at the same time.

BACKGROUND ART

Titanium alloys are representative lightweight metals, and are known as materials that create a high added value in various industrial fields based on their special characteristics that cannot be possessed by other materials.

Because their high specific strength and excellent corrosion resistance, titanium alloys may be utilized in variety of applications, including aerospace materials, chemical engineering materials, materials for in vivo use, electronic component materials, materials for sports equipment, and so on.

Among them, pure titanium, Ti-6Al-4V, Ti-6Al-7Nb and Ti—Ni alloys, etc., are used in vivo. However, these metals have a problem in that the elastic modulus is excessively higher than that of human body, resulting in the occurrence of the stress shielding phenomenon in which low stress is applied to bone tissue having a relatively low elastic modulus. Due to the stress shielding phenomenon, the human system recognizes the bond tissue, to which low stress is applied, as an unnecessary part, and thus dissolves the bone tissue by activating osteoclasts.

In addition, elements, such as aluminum (Al), vanadium (V) and nickel (Ni), are toxic to biological tissue. Thus, it has been required to have a biocompatible, low elastic modulus titanium alloy comprising elements such as titanium (Ti) niobium (Nb), zirconium (Zr), tantalum (Ta), etc., which are harmless to the human body.

In response to this requirement, alloys such as Ti-13Nb-13Zr and Ti-35Nb-5Ta-7Zr have been developed, which comprise biocompatible elements such as titanium (Ti), niobium (Nb), zirconium (Zr), tantalum (Ta) and the like and, at the same time, have low elastic modulus.

However, generally, as the elastic modulus of metals decreases, the strength thereof also decreases. For this reason, components made of such materials have very low fatigue resistance, and there is a limit to miniaturization of the components, so that the application of minimal invasive surgery that is very advantageous for patients will be limited.

Furthermore, materials for orthopedic or orthodontic use are required to have high superelastic elongation in addition to low elastic modulus and high strength properties.

In addition, materials exhibiting ultrahigh strength, ultralow elastic modulus and superelastic properties at the same time may be used as structural materials and the like for flexible displays and wearable devices, which are future technologies.

Meanwhile, metal materials that are used in flexible displays and wearable devices are required to have maximized flexibility without containing nickel (Ni) known to cause skin and allergic reactions. Flexibility can be largely divided into the flexibility of material itself and structural flexibility. To enhance the flexibility of material itself, the material should exhibit nonlinear elastic deformation and have stable superelastic and ultralow elastic modulus properties, so that the material can be bent even by a small force.

In addition, structural flexibility increases as the thickness of the material decreases. When a material has low strength, the fatigue resistance of the material itself significantly decreases as the thickness thereof decreases. For this reason, it is required to increase the strength of materials.

Thus, it can be seen that the properties required for metals that are used in flexible displays and wearable devices are the same as those of metals for in vivo use. Considering that the industry of flexible displays and wearable devices is a high-tech, high-value-added industry, it is required to develop a titanium alloy which is biocompatible and, at the same time, has ultrahigh strength, ultralow elastic modulus and stable superelastic properties.

In connection with this, U.S. Pat. No. 7,261,782 (Patent Document 1) discloses a titanium alloy exhibiting nonlinear elastic deformation and having superelastic properties.

However, the titanium alloy disclosed in Patent Document 1 has a disadvantage in that the elastic modulus of the titanium alloy decreases rapidly as the strength thereof increases. Furthermore, the titanium alloy contains vanadium (V) toxic to the human body, and thus is hard to apply as titanium for in vivo use. In addition, it has problems in that it contains tantalum (Ta) having a very high melting point of 3,017° C., and thus needs to be melted repeatedly, resulting in an increase in the production cost, and in that the non-uniformity of the alloy composition frequently occurs due to heavy tantalum (Ta). Additionally, even when the oxygen content of the titanium alloy changes by a very small amount, the superelastic elongation of the titanium alloy changes rapidly, making it difficult to uniformly control the properties of the titanium alloy in production of a large amount of the titanium alloy.

Furthermore, U.S. Pat. No. 7,722,805 (Patent Document 2) discloses a titanium alloy exhibiting ultralow elasticity and high-strength properties.

However, the titanium alloy disclosed in Patent Document 2 has advantages in that when oxygen is added in order to increase the strength of the titanium alloy, the superelastic elongation thereof decreases rapidly, and in that tin (Sn) that is added as a major alloying element has very low corrosion resistance in vivo compared to titanium (Ti), niobium (Nb), zirconium (Zr) and thus like, and thus is easily corroded.

Furthermore, there is a disadvantage in that an additional heat-treatment process is required to increase the strength of the titanium, and thus the production cost is increased by the complicated process. In addition, even when the oxygen content of the titanium alloy changes by a very small amount, the superelastic elongation of the titanium alloy changes rapidly, making it difficult to uniformly control the properties of the titanium alloy in production of a large amount of the titanium alloy.

Patent Document 1 also discloses a Ti—Nb—Zr-O-based alloy which does not contain tantalum (Ta) and tin (Sn).

However, this alloy has problems in that, when the oxygen (O) content of the alloy is excessively low, the alloy will not show high strength, and when the oxygen (O) content is excessively high, the elastic modulus of the alloy will increase rapidly, making it difficult to form the alloy into a certain shape, and the variation in elastic strain of the alloy will increase, making it difficult to produce the alloy in large amounts.

DISCLOSURE Technical Problem

It is an object of the present invention to provide a titanium alloy which can exhibit high strength, ultralow elastic modulus, and excellent superelastic elongation which is stable against a change in the oxygen content thereof, without having to contain alloying elements that are toxic to the human body, or that have low corrosion resistance in vivo, or that are heavy in weight while having a high melting point.

Technical Solution

To achieve the above object, the present invention provides a titanium alloy comprising Nb, Zr and O as alloying elements, with the remainder being Ti and inevitable impurities, the titanium alloy having an electron/atom (e/a) ratio of 4.17-4.22, a Mo equivalent (Mo_(eq)) of 7.50-9.72, and an Al equivalent (Al_(eq)) of 1.42-14.53.

In the present invention, the electron/atom (e/a) ratio may be 4.19-4.21, the Mo equivalent (Mo_(eq)) may be 8.19-9.03, and the Al equivalent (Al_(eq)) may be.

The titanium alloy may comprise 30-34 mass % of Nb, 5.7-9.7 mass % of Zr, and 0.03-1.0 mass % of O.

The coefficient of correlation of a decrease in superelastic elongation (%) to an increase in oxygen concentration after cold working of the titanium alloy may be −0.5 (%/mass %) or more.

The titanium alloy may have a superelastic elongation of 2.5% or more after cold working.

The titanium alloy may have an elastic modulus of 60 GPa or less and a tensile strength of 1000 MPa or more after cold working.

The titanium alloy may have a tensile strength (MPa)/average elastic modulus (GPa) ratio of 0.020 or more after cold working.

Advantageous Effects

The titanium alloy according to the present invention can maintain its ultralow elastic modulus together with its high strength. Thus, as shown in FIG. 1, the elastic elongation of the titanium alloy can be significantly increased. Accordingly, the titanium alloy can be applied to various fields, including flexible displays, wearable devices, aerospace fields, power generation fields, living goods, etc., which require excellent superelastic properties.

Furthermore, the titanium alloy according to the present invention shows a very small variation in the superelastic elongation with a change in the oxygen content thereof. Thus, it can exhibit uniform properties despite a variation oxygen content between various portions, which inevitably occurs when it is produced in large amounts. Therefore, it is advantageous in terms of mass production.

Moreover, the titanium alloy according to the present invention does not contain elements toxic to the human body, such as aluminum (Al), vanadium (V) or nickel (Ni), and also does riot contain tin (Sn) having low corrosion resistance in vivo. Thus, it can also be properly used in vivo.

In addition, the titanium alloy according to the present invention exhibits high strength and ultralow elasticity without having to contain tantalum (Ta), which is advantageous for achieving low elasticity but is heavy in weight and has a high melting point. Thus, it can be easily produced and substantially does not have composition non-uniformity, compared to conventional titanium alloys containing tantalum (Ta).

Additionally, the titanium alloy according to the present invention has excellent formability, and thus can be cold-formed by at least 90%.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the superelastic properties of a titanium alloy according to the present invention in comparison with a conventional titanium alloy.

FIG. 2 compares the strength/elastic modulus values of Ti—Nb—Zr—(O) alloys with varying electron/atom (e/a) ratios.

FIG. 3 compares the strength/elastic modulus values of Ti—Nb—Zr—(O) alloys with varying Mo_(eq) values.

FIG. 4 compares the strength/elastic modulus values of Ti—Nb—Zr—(O) alloys with varying Al_(eq) values.

BEST MODE

Hereinafter, titanium alloy according to the present invention, which exhibits nonlinear elastic deformation and has ultrahigh strength, ultralow elastic modulus and stable superelastic properties at the same time, will be described with reference to FIGS. 1 to 4.

The terms and words used in the specification and claims should not be interpreted as being limited to typical meanings or dictionary definitions, but should be interpreted as having meanings and concepts relevant to the technical scope of the present invention, based on the principle according to which the inventors can appropriately define the concept of the terms to describe their invention in the best manner.

Accordingly, it should be understood that the embodiments described in the specification and the configurations shown in the drawings are merely examples and do not represent all of the technical spirits of the invention, and thus there may be various equivalents and modifications capable of replacing them at the time of filing of the present invention.

FIG. 1 illustrates a difference in superelastic properties between a titanium alloy according to the present invention and a conventional titanium alloy.

As described above, in order to achieve a titanium alloy having ultralow elastic modulus and high strength without having to contain alloying elements such as aluminum (Al), vanadium (V) or nickel (Ni), which are harmful to the human body, an alloying element such as tin (Sn) having low corrosion resistance in vivo, and an alloying element such as tantalum (Ta) which is heavy in weight and has a very high melting point, a Ti—Nb—Zr alloy was developed.

When oxygen (O) that is a solid-solution strengthening element is added in order to increase the strength of the Ti—Nb—Zr alloy, the strength will increase while the elastic modulus of the alloy will also increase rapidly.

Thus, as shown in FIG. 1, as the strength increases, the elastic modulus changes greatly. When the conventional titanium alloy is produced in large amounts, a variation in oxygen content between different portions inevitably occurs, and for this reason, the conventional titanium alloy hardly exhibits uniform physical properties, and also hardly exhibits both high strength and ultralow elastic modulus.

As shown in FIG. 1, when the titanium alloy exhibits both high strength and ultralow elastic modulus, the elastic strain thereof significantly increases and a variation in the elastic strain also decreases.

The present inventors have made efforts to develop a titanium alloy which can exhibit both high strength and ultralow elastic modulus while showing no great variation in the elastic strain thereof even when the solid-solution strengthening element oxygen is added to the conventional Ti—Nb—Zr-based alloy. As a result, the present inventors have found that, when the electron/atom (e/a) ratio, beta-phase stabilizing element Mo equivalent (Mo_(eq)) and alpha-phase stabilizing element Al equivalent (Al_(eq)) of the titanium alloy, which were not taken into consideration in the design of the conventional titanium alloy, are all maintained in certain ranges, the titanium alloy can exhibit all the above-described properties, thereby completing the present invention.

In the present invention, the electron/atom (e/a) ratio, the Mo equivalent (Mo_(eq)) and the Al equivalent (Al_(eq)) can be calculated using the following equations:

Electron/atom (e/a) ratio=Ti (atom %)×0.04+Nb (atom %)×0.05+Zr (atom %)×0.04;   Equation 1

Mo equivalent (Mo_(eq))=Nb (mass %)/3.6;   Equation 2

Al equivalent (Al_(eq))=Zr (mass %)/6+O (mass %)×10.   Equation 3

The titanium alloy according to the present invention comprises Nb, Zr and O as alloying elements, with the remainder being Ti and inevitable impurities.

Specifically, the titanium alloy according to the present invention does not contain elements such as aluminum (Al), vanadium (V) or nickel (Ni), which are toxic to the human body, tin (Sn) which has low corrosion resistance in vivo, and tantalum (Ta) which has a very high melting point and is heavy in weight.

If the electron/atom (e/a) ratio is lower than 4.17 or more than 4.22, it is impossible to achieve a superelastic elongation of 2% or more, an elastic modulus of 60 GPa or less and a tensile strength of 1000 MPa or more at the same time. For this reason, the electron/atom (e/a) ratio is preferably 4.17-4.22, more preferably 4.19-4.21.

If the Mo equivalent (Mo_(eq)) is lower than 7.50 or more than 9.72, it is impossible to achieve a superelastic elongation of 2% or more, an elastic modulus of 60 GPa or less and a tensile strength of 1000 MPa or more at the same time. For this reason, the Mo equivalent (Mo_(eq)) is preferably 7.50-9.72, more preferably 8.19-9.03.

If the Al equivalent (Al_(eq)) is lower than 1.42 or more than 14.53, it is impossible to achieve a superelastic elongation of 2% or more, an elastic modulus of 60 GPa or less and a tensile strength of 1000 MPa or more at the same time. For this reason, the Al equivalent (Al_(eq)) is preferably 1.42-14.53, more preferably 1.60-10.78.

In order to maintain the electron/atom (e/a) ratio, the beta-phase stabilizing element Mo equivalent (Mo_(eq)) and the alpha-phase stabilizing element Al equivalent (Al_(eq)) in the above-described ranges, the composition of the Ti—Nb—Zr—O alloy preferably comprises 30-34 mass % of Nb, 5.7-9.7 mass % of Zr, and 0.03-1.0 mass % of O.

In addition, the titanium alloy according to the present invention may also comprise tantalum in an small amount (1 mass % or less) within a range that does not impair the melting and uniformity of the alloy.

The titanium alloy according to the present invention may contain impurities that are inevitably incorporated in raw materials or in a production process. The contents of these impurities is controlled to 1 mass % or less, preferably 0.1 mass % or less, more preferably 0.01 mass % or less.

Hereinafter, the present invention will be described. in further detail with reference to titanium alloys according to preferred examples of the present invention and comparative examples.

Each of titanium alloys according to Examples 1 to 7 of the present invention and Comparative Examples 1 to 4 was obtained by preparing a titanium alloy melt having the composition shown in Table 1 below, casting the titanium alloy melt into a billet, hot-rolling the billet at 1000° C., cooling the hot-rolled billet to room temperature, and then cold-rolling the billet at a reduction in area of 90%.

TABLE 1 Content (mass %) Ti Nb Zr O Remarks e/a Mo_(eq) Al_(eq) Example 1 62.44 29.5 8 0.06 Cold 4.19 8.19 1.93 Example 2 59.45 32.5 8 0.05 working 4.21 9.03 1.83 Comparative 61.99 30 8 0.01 4.19 8.33 1.43 Example 3 Example 3 61.95 30 8 0.05 4.19 8.33 1.83 61.69 30 8 0.31 4.19 8.33 4.43 61.42 30 8 0.58 4.19 8.33 7.13 61.11 30 8 0.89 4.19 8.33 10.23 Comparative 60.68 30 8 1.32 4.19 8.33 14.53 Example 2 Example 4 61.44 32.5 6 0.06 4.21 9.03 1.60 Example 5 57.95 32.5 9.5 0.05 4.21 9.03 2.08 57.72 32.5 9.5 0.28 4.21 9.03 4.38 57.39 32.5 9.5 0.61 4.21 9.03 7.68 57.08 32.5 9.5 0.92 4.21 9.03 10.78 Example 6 63.94 29.5 6.5 0.06 4.18 8.19 1.68 63.72 29.5 6.5 0.28 4.18 8.19 3.88 63.4 29.5 6.5 0.6 4.19 8.19 7.08 63.12 29.5 6.5 0.88 4.19 8.19 9.88 Example 7 60.94 29.5 9.5 0.06 4.19 8.19 2.18 Comparative 67.94 27 5 0.06 4.16 7.50 1.43 Example 3 Comparative 55.45 35 9.5 0.05 4.23 9.72 2.08 Example 4 Comparative 55.6 35 9 0.4 4.23 9.72 5.50 Example 5 Comparative 86 5 9 0 4.03 1.39 1.50 Example 6 81 10 9 0 4.06 2.78 1.50 76 15 9 0 4.09 4.17 1.50 Comparative 51.4 22.3 26.3 0 4.15 6.19 4.38 Example 7 50.3 23.6 26.1 0 4.16 6.56 4.35 49.2 24.9 25.9 0 4.17 6.92 4.32 48.1 26.2 25.7 0 4.18 7.28 4.28 Comparative 73.93 13 13 0.07 Aging 4.08 3.61 2.87 Example 8 treatment Comparative 55.75 35 9 0.25 4.23 9.72 4.00 Example 9 Comparative 79 8 13 0 Solution 4.05 2.22 2.17 Example 10 69 18 13 0 treatment 4.11 5.00 2.17 51.8 41.1 7.1 0 4.28 11.42 1.18 Comparative 41 34 25 0 4.24 9.44 4.17 Example 11 38 30 32 0 4.22 8.33 5.33 36.6 28 35.4 0 4.21 7.78 5.90 34.5 24.8 40.7 0 4.19 6.89 6.78

In addition, the titanium alloys according to Comparative Examples 5 to 11 as shown in Table 1 above and the mechanical properties shown in Table 2 below are those disclosed in the following patent documents or publications. Furthermore, the electron/atom ratio (e/a), molybdenum equivalent and aluminum equivalent shown in Table 1 above are values calculated based on the disclosed compositions.

Comparative Example 5: Korean Patent Application Publication No. 2002-0026891.

Comparative Example 6: Q. Liu et al., Progress to Natural Science: Materials International, vol 23(6) (2013) pp. 562-565.

Comparative Example 7: H. Tobe et al., Materials Transactions, vol 50 (2009) pp. 2721-2725.

Comparative Example 8: C. H. Park et al., Materials Science and Engineering A, vol 527 (2010) pp. 4914-4919.

Comparative Example 9: Korean Patent Application Publication No. 2003-0061007.

Comparative Example 10: S. Schneider et al., Materials Research, vol 8 (2005) pp. 435-438.

Comparative Example 11: S. Ozan et al., Acta Biomaterialia, vol 20 (2015) pp. 176-187.

As shown in Table 1 above, the titanium alloys according to Examples 1 to 7 of the present invention had an electron/atom (e/a) ratio in the range of 4.17-4.22, a Mo equivalent (Mo_(eq)) in the range of 7.50-9.50, and an Al equivalent (Al_(eq)) in the range of 1.45-14.53.

On the other hand, the titanium alloys according to Comparative Examples 1 to 11 did not contain oxygen (O) as an essential element or had an electron/atom (e/a) ratio out of the range of 4.17-4.22, a Mo equivalent (Mo_(eq)) out of the range of 7.50-9.50, and an Al equivalent (Al_(eq)) out of the range of 1.45-14.53.

The compositions shown in Table 1 above were subjected to subsequent working and heat treatment, and the mechanical properties of the resulting titanium alloys were evaluated. The results of the evaluation are summarized in Table 2 below.

TABLE 2 Tensile Elastic Superelastic Tensile strength modulus elongation strength/elastic No. (MPa) (GPa) (%) modulus Example 1 1110 35 2.7 0.0317 Example 2 1105 34 2.8 0.0325 Comparative 765 32 2.1 0.0239 Example 1 Example 3 1062 32 2.9 0.0332 1134 35 2.8 0.0324 1243 38 2.8 0.0327 1349 47 2.5 0.0287 Comparative 1560 70 1.8 0.0223 Example 2 Example 4 1071 34 2.7 0.0315 Example 5 1056 33 2.8 0.0320 1120 36 2.7 0.0311 1163 38 2.6 0.0306 1377 49 2.5 0.0281 Example 6 1027 32 2.8 0.0321 1014 32 2.7 0.0317 1125 37 2.6 0.0304 1217 43 2.5 0.0283 Example 7 1114 37 2.6 0.0301 Comparative 1034 44 2 0.0235 Example 3 Comparative 977 45 1.9 0.0217 Example 4 Comparative 950 47 — 0.0202 Example 5 Comparative 904 62.3 — 0.0145 Example 6 857 54.4 — 0.0158 854 38.8 — 0.0220 Comparative 982 51 — 0.0193 Example 7 1008 52 — 0.0194 1005 52 — 0.0193 848 58 — 0.0146 Comparative 902 80 — 0.0113 Example 8 Comparative 1555 85 1.8 0.0183 Example 9 Comparative 763 88.8 — 0.0086 Example 10 698 70.4 — 0.0099 499 64.7 — 0.0077 Comparative 839 62 1.31 0.0135 Example 11 794 65 1.2 0.0122 755 64 1.14 0.0118 704 63 1.08 0.0112

The results summarized in Table 2 above are shown in FIGS. 2 to 4.

As shown in Table 2 above, the titanium alloys according to Examples 1 to 7 of the present invention exhibited a tensile strength of 1000 MPa or more and an elastic modulus of 50 GPa or less and, at the same time, exhibited a superelastic elongation of 2.5% or more. Namely, the titanium alloys according to the present invention exhibited high strength, ultralow elastic modulus and excellent superelastic elongation, which could not be exhibited by conventional titanium alloys.

In addition, as shown in Examples 3, 5 and 6 of the present invention, the rate of decrease in the superelastic elongation was very low even when the oxygen content increased, and the rate of the decrease was as slow as −0.5 or more. Namely, the titanium alloys according to the Examples of the present invention can exhibit superelastic properties that are stable against a change in the oxygen content.

Meanwhile, Comparative Example 1 had Nb and Zr contents similar to those of Example 1, but had a low oxygen content, and thus failed to exhibit the properties obtained in Examples 1 to 7 of the present invention. Comparative Example 2 had Nb and Zr contents similar to those of Example 3, but had an excessively high oxygen content, and thus failed to exhibit the properties obtained in Examples 1 to 7 of the present invention.

In addition, the titanium alloys according to Comparative Examples 3 to 11 had a Nb or Zr content different from those of the Examples of the present invention, and as a result, exhibited a low strength, an excessively high elastic modulus or a low superelastic elongation, compared to those of Examples 1 to 7 of the present invention.

FIGS. 2 to 4 diagrammatically show the results shown in Table 2 above.

As shown in FIG. 2, the electron/atom (e/a) ratios of the titanium alloys according to Examples 1 to 7 were between about 4.175 and about 4.225, and the titanium alloys of Examples 1 to 7, which had such electron/atom (e/a) ratios, showed tensile strength/elastic modulus ratios higher than those of the Comparative Examples, which did not have such electron/atom (e/a) ratios.

Furthermore, as shown FIG. 3, the titanium alloys according to Examples 1 to 7 had a Mo equivalent (Mo_(eq)) between 8 and 9, and showed tensile strength/elastic modulus ratios higher than those of the Comparative Examples, which had Mo equivalents out of this Mo equivalent range.

In addition, as shown FIG. 4, the titanium alloys according to Examples 1 to 7 had an Al equivalent (Al_(eq)) between 1.75 and 11, and showed tensile strength/elastic modulus ratios higher than those of the Comparative Examples, which had Al equivalents out of this Al equivalent range.

As described above, the alloys of Examples 1 to 7 of the present invention, which satisfy the above-described three conditions, can exhibit high strength, ultralow elastic modulus and superelastic elongation at the same time, but alloys which do not satisfy such conditions do not exhibit at least one of high strength, ultralow elastic modulus and superelastic elongation. 

1. A titanium alloy comprising Nb, Zr and O as alloying elements, with a remainder being Ti and inevitable impurities, the titanium alloy having an electron/atom (e/a) ratio of 4.17-4.22, a Mo equivalent (Mo_(eq)) of 7.50-9.72, and an Al equivalent (Al_(eq)) of 1.42-14.53.
 2. The titanium alloy of claim 1, having an electron/atom (e/a) ratio of 4.19-4.21, a Mo equivalent (Mo_(eq)) of 8.19-9.03, and an Al equivalent (Al_(eq)) of 1.60-10.78.
 3. The titanium alloy of claim 1, comprising 30-34 mass % of Nb, 5.7-9.7 mass % of Zr, and 0.03-1.0 mass % of O.
 4. The titanium alloy of claim 1, wherein a coefficient of correlation of a decrease in superelastic elongation (%) to an increase in oxygen concentration after cold working of the titanium alloy is −0.5 (%/mass %) or more.
 5. The titanium alloy of claim 1, having a superelastic elongation of 2.5% or more after cold working.
 6. The titanium alloy of claim 1, having an elastic modulus of 60 GPa or less and a tensile strength of 1000 MPa or more after cold working.
 7. The titanium alloy of claim 1, having a tensile strength (MPa)/average elastic modulus (GPa) ratio of 0.020 or more after cold working. 